Itemized Deductions or the Standard Deduction can only reduce your income to zero and not create a refund. Then you only get back any withholding taken out.
TurboTax starts off using the Standard deduction, so until your Itemized Deductions are higher than the Standard deduction your refund or tax due would not change. Or it will just change a little at a time as you increase your deductions over the standard amount.
And by increasing your deductions it will decease your income and you may not be getting as many credits as before. Also after you reduce your income to zero there is no more refund to get back but you still may owe for other things like self employment tax or the 10% early withdrawal penalty from 401k or IRA accounts.
Or you are subject to the AMT tax. AMT is Alternative minimum tax. See http://www.irs.gov/taxtopics/tc556.html
and https://ttlc.intuit.com/questions/1899184-what-is-the-alternative-minimum-tax-amt-and-do-i-have-to-p...
Or you have self employment tax. You can't reduce the SE tax. If all your tax is self-employment tax rather than income tax, any deductions or an IRA contribution will not lower it. Or you owe something else like a 10% early withdrawal penalty from your 401k or IRA. Or the penalty for not having Health insurance or you need to pay back some of the subsidy. Those are not affected by adding deductions or contributions.
